This invention relates generally to a method for determining structural integrity of a material and for detecting subsurface voids within one or more layers of the material. More specifically, the present invention is directed towards using an easily determinable, substantially constant surface wave velocity as means for determining structural integrity, layer thickness, set time and subsurface anomalies within curing or cured materials. Substantially constant surface wave velocity can be measured in situ at different times during which the material is in the process of changing from a soft to hard state, or from a hard to soft state.
There has always been a need for determining the structural integrity of materials (i.e., asphalt, concrete, paint, glue, etc.) that change state or change density over time. Materials which are capable of being cured often exhibit different structural characteristics over time. Conventional structural tests are commonly done by taking representative samples of the material and applying compression force to the samples to determine the points in which the samples break. Samples are often withdrawn from the material after the material has cured or dried. A driver is applied to the sample and measurements are taken on how far the sample is compressed before it breaks or crumbles. The compression load and distance are then used to give the engineer an estimate of the strength of the sample. If the sample does not meet the engineer's specifications, the material is removed and a new material, hopefully within the engineer's specifications, will then be constructed.
The cylinder compression test requires that a sample be withdrawn from the material and that the sample be destroyed. Thus, a sample must be cut from the material and after the test is done, the area where the sample has been removed must be patched or filled-in. Often times, a sample is taken from a remote area so that the fill-in will not detrimentally affect heavily used, critical areas. For example, this is what occurs when highway engineers remove pavement samples from highway curbs or roadway edges rather than the middle of the highway. The problem with removing samples is that it takes time and a great deal of effort to cut the sample from the material body. In order to eliminate the time it takes to withdraw the sample, seperate samples are often made apart from the material itself. For example, a separate sample of concrete is often poured into test cylinders apart from the structure itself. Therefore, rather than removing a sample from a concrete pillar, a separate sample is poured and tested. However, the separately constructed samples are not always indicative of the material body itself. Concrete within large pillars can have inherently different characteristics than concrete in small test cylinders. Even if the cylinder sample and the pillar was poured at the same time, poured from the same "batch", or was on-site conditioned and cured under the same environmental conditions, smaller-mass test cylinders may not have the same structural integrity as does the larger-mass material (e.g., pillar). Therefore, testing of a sample of material cannot give the engineer a true and accurate measure of the material to which the engineer is really interested in measuring.
While cylinder tests analyze structural integrity of cured material, penetration resistance tests are used to analyze materials during the curing process. For example, penetration resistance of fresh concrete is the most common test used to determine to what extent the concrete has cured or has "set". Like cylinder tests, penetration resistance tests often require a sample of the material under test. A penetrometer is forced into the sample and the amount of force needed to cause 1 inch of penetration is recorded. The amount of force is then divided by the cross-sectional area of the penetrometer to calculate the penetration resistance. Initial set is defined as the time in which penetration resistance is equal to 500 lbs./sq. in (psi), wherein final set is when penetration resistance is 4000 psi. Set time is important in that it indicates the amount of curing taking place within the material, and the earliest time when it may be possible to make physical contact with the material.
Although set time can be tested prior to when the material being fully cured, structural integrity is typically tested not during, but after the material has cured. This requires that the material be hardened or stiff before the cylinder compression testing can begin. If the tests indicate poor structural integrity or a weakness within the sample, then the hardened material must be removed. Removing defective material after it becomes hardened can be very expensive when large amounts of cured material have been deposited.
In order to perform testing before the material has cured, seismic surface waves are typically used to predict subsurface properties. Spectral-Analysis-of-Surface-Waves (SASW) has been the preferred method used to predict subsurface properties. Although conventional SASW methods perform a non-intrusive, in situ testing upon the material either before or after curing is complete, conventional SASW methods can only predict a limited number of subsurface properties. Using a derived, substantially constant surface wave velocity, conventional SASW methods typically measure only Young's modulus or stiffness of the material. However, Young's modulus, in and of itself, is not an accurate indicator of the material's structural integrity, nor is Young's modulus an accurate predictor of set time. Moreover, Young's modulus cannot locate and identify subsurface anomalies or voids. While SASW can replace conventional cylinder compression and penetration resistance test as a preferred method of non-intrusively measuring Young's modulus, conventional SASW methods do not measure structural integrity (i.e., strength, or dampening properties of the material), set time layer thickness and, most importantly, cannot detect the presence of voids. Simply measuring Young's modulus will not provide the engineer with the more desirable measurements such as integrity, set time layer thickness and subsurface voids or anomalies.